Collimator and Detector Based Medical Imaging Systems

ABSTRACT

A medical imaging system includes a first collimator configured to filter radiation emitted from a subject, a first detector configured to detect radiation that has passed through the first collimator, a second collimator configured to filter radiation emitted from the subject, wherein the first collimator partially blocks a field of view (FOV) of the second collimator, and a second detector configured to detect radiation that has passed through the second collimator.

PRIORITY

This claims the benefits of U.S. Provisional Patent Application No.62/758,183 filed Nov. 9, 2018 and U.S. Provisional Patent ApplicationNo. 62/760,659 filed Nov. 13, 2018, the entire disclosure of which isincorporated herein by reference.

BACKGROUND

In molecular medical imaging, sometimes known as nuclear medicine,images representing radiopharmaceutical distributions may be generatedfor medical diagnosis. Prior to imaging, radiopharmaceuticals areinjected into an imaging subject such as a patient. Theradiopharmaceuticals emit radioactive photons, which can penetratethrough the body to be detected by a photon detector. Based oninformation from the received photons, the photon detector may thendetermine the distribution of the radiopharmaceuticals inside thepatient. Their distribution represents the physiological function of thepatient, and therefore images of their distribution provide valuableclinical information for diagnosis of a variety of diseases andconditions such as those in cardiology, oncology, neurology, etc.

A collimator is a device that guides photon path. In molecular imaging,photons may originate from unknown locations inside a subject, unlike inX-ray or CT where photons are emitted from a known source (or sources)position. Without collimators, photons from all directions may berecorded by gamma detectors, and image reconstruction may becomedifficult. Therefore, collimators are employed to guide possible photonpaths so that images can be reconstructed, similar to the role of lensin a photography camera. Although existing collimator and detectorimaging systems have been generally adequate for their intendedpurposes, they have not been entirely satisfactory in all respects. Forexample, existing imaging systems often have limited imaging sensitivityor resolution and often suffer from heavy noises. Therefore,improvements on collimator and detector imaging systems are desired.

SUMMARY

According to various embodiments, the present disclosure provides amedical imaging system, including a first collimator configured tofilter radiation emitted from a subject, a first detector configured todetect radiation that has passed through the first collimator, a secondcollimator configured to filter radiation emitted from the subject,wherein the first collimator partially blocks a field of view (FOV) ofthe second collimator, and a second detector configured to detectradiation that has passed through the second collimator.

In some embodiments, the second detector and the second collimator areconfigured to move together. In some embodiments, the first collimatorincludes first openings for passing through radiation with a firstaspect ratio of height to width, and the second collimator includessecond openings for passing through radiation with a second aspect ratioof height to width. The first aspect ratio is higher than the secondaspect ratio. In some embodiments, the first openings of the firstcollimator include parallel holes. In some embodiments, the secondopenings of the second collimator include multiple pinholes in a plate.In some embodiments, the number of the pinholes on the second collimatoris 11 or greater. In some embodiments, the second collimator ispositioned at least 3 cm away from the first collimator. In someembodiments, the first collimator is positioned on a peripheral of theFOV of the second collimator. In some embodiments, the first collimatorincludes a superior portion and an inferior portion which are separatedby an empty portion, and the empty portion is outside a FOV of the firstcollimator but within the FOV of the second collimator. In someembodiments, the first collimator further includes left and rightportions, each of which connects the superior and inferior portions. Theleft portion, the right portion, the superior portion, and the inferiorportion together surround the empty portion. In some embodiments, a sizeof the first detector is such that the first detector is fullypositioned between the first and second collimators. In someembodiments, the first detector is based on at least one of: cadmiumtelluride (CdTe), cadmium zinc telluride (CZT), and high puritygermanium (HPGe). In some embodiments, the medical imaging systemfurther includes an image processor configured to reconstruct a medicalimage of the subject based on radiation detected by both the firstdetector and the second detector, and a monitor configured to displaythe reconstructed medical image.

According to various embodiments, the present disclosure also provides amolecular breast imaging (MBI) system for examining a breast on a chestof a patient. The MBI system includes a first collimator configured forplacement proximal to the chest, and a second collimator configured forplacement distal to the chest. The first and second collimators areplaced on a same side of the breast, and the second collimator ispositioned further away from the breast than the first collimator. Insome embodiments, the first and second collimators include first andsecond openings, respectively, for passing through radiation emittedfrom the patient. The first and second openings have first and secondaspect ratios, respectively, of height to width, and the first aspectratio is higher than the second aspect ratio. In some embodiments, theMBI system further includes a third collimator for placement proximal tothe chest, and a fourth collimator for placement distal to the chest.The third collimator also includes openings for passing throughradiation with the first aspect ratio of height to width, and the fourthcollimator also includes openings for passing through radiation with thesecond aspect ratio of height to width. The third and fourth collimatorsare positioned on a same side of the breast that is opposite to the sideof the breast where the first and second collimators are placed on. Insome embodiments, the openings of the first collimator include parallelholes. In some embodiments, the openings of the second collimatorinclude multiple pinholes. In some embodiments, the MBI system furtherincludes a first detector configured to detect radiation that has passedthrough the first collimator, and a second detector configured to detectradiation that has passed through the second collimator. In someembodiments, the second collimator and the second detector areconfigured to move about the breast while maintaining a constantdistance between the second collimator and the second detector.

According to various embodiments, the present disclosure also provides amethod for examining a subject using molecular imaging. The methodincludes filtering, by a first collimator, photons emitted from thesubject, detecting, by a first detector, photons that pass through thefirst collimator, filtering, by a second collimator, photons emittedfrom the subject, detecting, by a second detector, photons that passthrough the second collimator, and reconstructing, by an imageprocessor, a medical image of the subject based on photons detected byboth the first and second detectors. In some embodiments, the first andsecond collimators are positioned such that their fields of viewpartially overlap in an overlapping region. In some embodiments, thereconstructing of the medical image includes reconstructing a firstpartial image based on photons that pass through the overlapping regionand are detected by the first detector, and reconstructing a secondpartial image based on photons detected by the second detector. Thereconstruction of the second partial image uses the first partial imageas an initial estimate for the overlapping volume. In some embodiments,the reconstruction of the second partial image further updates the firstpartial image using an iterative method.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIGS. 1A and 1B are schematic top and cross-sectional views,respectively, of an example imaging system according to various aspectsof the present disclosure.

FIG. 2 is a schematic cross-sectional view of part of an imaging systemaccording to various aspects of the present disclosure.

FIG. 3 is a schematic diagram of an example nuclear imaging systemaccording to various aspects of the present disclosure.

FIG. 4A is a schematic top view of one molecular breast imaging (MBI)system, and FIG. 4B is a cross-sectional view of two MBI systems shownin FIG. 4A, according to various aspects of the present disclosure.

FIG. 4C is a schematic top view of another MBI system, and FIG. 4D is across-sectional view of two MBI systems shown in FIG. 4C, according tovarious aspects of the present disclosure.

FIG. 5 is a flow chart of a method of examining a subject according tovarious aspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the disclosure.Specific examples of components and arrangements are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. Any alterations and furthermodifications to the described devices, systems, methods, and anyfurther application of the principles of the present disclosure arefully contemplated as would normally occur to one having ordinary skillin the art to which the disclosure relates. For example, the features,components, and/or steps described with respect to one embodiment may becombined with the features, components, and/or steps described withrespect to other embodiments of the present disclosure to form yetanother embodiment of a device, system, or method according to thepresent disclosure even though such a combination is not explicitlyshown. In addition, the present disclosure may repeat reference numeralsand/or letters in the various examples. This repetition is forsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Moreover, a feature on, connected to, and/or coupled to another featurein the present disclosure that follows may include embodiments in whichthe features are in direct contact, and may also include embodiments inwhich additional features may interpose the features, such that thefeatures may not be in direct contact. In addition, spatially relativeterms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,”“over,” “below,” “beneath,” “up,” “down,” “top,” “bottom,” etc., as wellas derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,”etc.) are used for ease of the present disclosure of one featuresrelationship to another feature. The spatially relative terms areintended to cover different orientations of the device including thefeatures. Still further, when a number or a range of numbers isdescribed with “about,” “approximate,” and the like, the term isintended to encompass numbers that are within a reasonable rangeincluding the number described, such as within +/−10% of the numberdescribed or other values as understood by person skilled in the art.For example, the term “about 5 cm” encompasses the dimension range from4.5 cm to 5.5 cm.

The present disclosure is generally related to the field of medicalimaging, and more particularly to the design of collimator and detectorused in molecular imaging systems.

In molecular medical imaging systems, collimator and detector work intandem to generate images that represent radiopharmaceuticaldistributions within a subject. However, existing collimator anddetector designs suffer from various issues. For example, conventionallyonly one type of collimator is used to guide photons, but one collimatortype is often insufficient to capture enough useful photons emitted froma patient, which leads to limited imaging sensitivity or resolution. Foranother example, sometimes a collimator-based imaging system is used toimage a certain part of a patient, such as a breast. In this case, toincrease imaging sensitivity, the collimator often has holes or otherpassthrough features that allow wide angles of incident photons to passthrough. But with such a design, the collimator may also allow undesiredphotons (photons emitted from other parts of the patient's body) to passthrough, thereby increasing noise.

The present disclosure provides new collimator and detector systemdesigns to solve the problems in conventional systems and to improveperformance. According to some embodiments, a molecular imaging systemuses multiple types of collimators and detectors (instead of a singlecollimator type and a single detector type) on the same imager head(positioned at the same side of a patient). In an example system, afirst collimator is configured to filter radiation emitted from asubject, and a first detector is coupled to (e.g., fixed to or movablyconnected to) the first collimator to detect radiation that has passedthrough the first collimator. A second collimator, which is of a typedifferent from the first collimator, is positioned on the same side ofthe subject as the first collimator but further away from the subjectthan the first collimator and configured to filter radiation emittedfrom the subject, and a second detector is coupled to (e.g., fixed to ormovably connected to) the second collimator to detect radiation that haspassed through the second collimator. The first collimator may bedisposed on the peripheral of a field of view (FOV) of the secondcollimator. As a result, the first collimator partially restricts orblocks the FOV of the second collimator, but its empty center portionallows the second collimator to receive photons therethrough. Such aconfiguration allows the first collimator and detector tandem to pick upperipheral photons, which increases imaging sensitivity, while reducingnoises to be received by the second collimator. Therefore, systemperformance may be improved.

FIGS. 1A and 1B are schematic top and cross-sectional views,respectively, of an example imaging system 100, which may be used tomedically examine or treat a subject such as patient 102. Imaging system100 is a hybrid system including two imaging modules or parts (calledimagers herein) 110 and 120. Imager 110 includes first collimator 112and first detector 114, and imager 120 includes second collimator 122and second detector 124. Imaging system 100 may include other parts suchas connectors that couple parts together (e.g., connecting imagers 110and 120 together), motors that cause parts to move, photon shieldingcomponents, a housing component that contains other parts, etc. Forexample, a coupling and shielding component 126 may connect collimator122 and detector 124 such that collimator 122 and detector 124 move(e.g., rotate) together, and prevent radiation (photons) from reachingdetector 124 through paths other than collimator 122. In otherembodiments, collimator 122 and detector 124 may move individually withrespect to each other. Notations “S” and “I” in FIGS. 1A and 1B standfor superior (head) and inferior (foot) positions, respectively.

When imaging system 100 is used to medically examine or treat patient102, which may be a human or an animal, one or more radiopharmaceuticalsmay be taken orally or injected into patient 102. Theradiopharmaceutical undergoes nuclear decay and may emit, eitherdirectly or indirectly through annihilation, radiation (e.g., gammaphotons) at certain rate(s) and with characteristic energies. Detectors114 and 124 are placed near patient 102 to record or monitor emissions.In some embodiments, detectors 114 and 124 are organized in planarshapes to acquire data in a two-dimensional (2D) matrix format, whichmay be called projections. In other embodiments, detectors 114 and 124have curved surface shapes to increase a surface area for receivingphotons with incident angles greater than zero. Based on recordedinformation such as position, energy, and counts of such detectedevents, an image of radiopharmaceutical distribution may bereconstructed to study the status or function of certain body parts onpatient 102.

FIG. 2 is a schematic cross-sectional view of part of imaging system100. In an embodiment, collimator 112 is configured to filter radiationemitted from patient 102, and detector 114 is coupled to collimator 112to detect radiation that has passed through collimator 112. Collimator122 is positioned further away from patient 102 than collimator 112 andconfigured to filter radiation emitted from patient 102. As an example,collimator 122 may be placed at least several centimeters (e.g., 5 cm)away from a resting platform 130 for patient 102, while collimator 112may be placed as close as possible to the resting platform 130 forpatient 102 (e.g., 2 cm or less, such as a few millimeters). Thus,collimator 122 is placed below collimator 112 at a certain distance(e.g., at least 3 cm, or 5 cm, or 10 cm, or 15 cm, or 30 cm, or between1 to 2 meters, or greater than 2 meters), where different distances maysuit different applications or purposes. The distance between collimator112 and collimator 122 is needed for optimizing performance ofcollimator 122 such as improving resolution. The distance betweenpatient 102 and collimator 122 may also depend on the types of openingson collimators 112 and 122, which are described further below. Detector124 is coupled to collimator 122 to detect radiation that has passedthrough collimator 122. Collimator 112 may be empty in its centerportion and may be disposed along the perimeter of an FOV of collimator122. As a result, collimator 112 partially blocks the FOV of collimator122, but its empty center portion allows collimator 122 to receivephotons therethrough. Such a configuration allows collimator 112 anddetector 114 to pick up peripheral photons, which makes good use of thespace and increases imaging sensitivity, while reducing noises to bereceived by collimator 122. Therefore, system performance may beimproved. In an alternative embodiment, the center portion of collimator112 is not empty but nonetheless transparent to the radiation emittedfrom patient 102. This achieves the same or similar effects as having anempty center portion.

Collimators 112 and 122 are configured to filter radiation by blockingcertain photons and passing through other photons. Collimators 112 and122 may each be made of one or more radiation (e.g., photons) absorbingheavy medal(s), such as lead and/or tungsten. Collimator 112 hasopenings 113 built therein to allow some photons to pass through, andcollimator 122 has openings 123 built therein to allow some photons topass through. It should be understood that radiation or photon blockingor absorption by collimators disclosed herein does not require blockingof 100% of photons because a small percentage of photons (e.g., 5% orless) may still penetrate through the full thickness of the radiationabsorbing material. The number of escaping photons may decreaseexponentially with the thickness of a collimator. In other words,blocking (or other similar terms) means that substantially all of thephotons (e.g., 95% or more, or 98% or more) are absorbed by theradiation absorbing material.

As shown in FIG. 2, a photon may hit a top surface of collimator 112with an acceptable incident angle (denoted by symbol a in FIG. 2 as anangle between line A1 and the vertical direction Z where line A1 is thetravel direction of the photon and direction Z is the normal of the topsurface of collimator 112). If the incident angle is greater than apredetermined threshold value, the photon would be absorbed bycollimator 112 (note there are occasions where the photon cuts through aportion of collimator 112 adjacent the opening (e.g., a thin area on thesidewall of the opening)). Therefore, the acceptable incident angle arepresents the range of possible incident angles for photons to passthrough an opening 113 without cutting through a portion of collimator112.

In some embodiments, this threshold value ranges from 0° to about 2° orfrom 0° to about 10°. In an example, a LEHR (low energy high resolution)collimator has an opening diameter of about 1.11 mm and a length ofabout 24.04 mm, with an acceptable incident angle range of 0° to about2.64°. In another example, a GAP (general all purpose) collimator has anopening diameter of about 1.40 mm and a length of about 25.4 mm, with anacceptable incident angle range of 0° to about 3.15°). In yet anotherexample, a LEHS (low energy high sensitivity) collimator has an openingdiameter of about 2.54 mm, a length of about 24.04 mm, with anacceptable incident angle range of 0° to about 6.03°. The acceptableincident angle for collimator 112 is often less than 10°. Photons thatcan pass through collimator 112 is considered within an FOV ofcollimator 112 (denoted in FIG. 2 as FOV1, which is a space within linesA1 and A2).

Similarly, for collimator 122, if a photon hits a top surface ofcollimator 122 with an incident angle (denoted by symbol β in FIG. 2 anddefined as the travel direction of the photon and the normal of thecollimator 122) greater than a predetermined threshold value, the photonwould be absorbed by collimator 122 (note there are occasions where thephoton cuts through a portion of collimator 122 adjacent the opening(e.g., a thin area on the sidewall of the opening)). So only photonsincident on the surface of an opening 123 with incident angles less thanthe threshold value can possibly pass through opening 123. In someembodiments, this threshold value ranges from 0° to about 15°, or 0° toabout to 75°. For example, an opening with a diameter of about 1.0 mmand a length of about 3.0 mm has an acceptable incident angle of about18.43°, and an opening with a diameter of about 3.0 mm and a length ofabout 1.5 mm has an acceptable incident angle of about 63.43°. Theacceptable incident angle for collimator 122 is greater than that forcollimator 112 (e.g., greater than 15°). Photons that can pass throughcollimator 122 is considered within an FOV of collimator 122 (denoted inFIG. 2 as FOV2, which is a space within lines B1 and B2). In someembodiments, openings 113 and/or 123 have irregular shapes. For example,an opening 113 may have a different width from another opening 113, andan opening 123 may have a different width from another opening 123. Anopening 113 or 123 itself may have a varying width. In such cases, theincident angle (α or β) may be determined using flexible methods thatare consistent with principles disclosed herein (i.e., the goal is forcollimator 112 to pass photons that have a narrower range of incidentangles than collimator 122).

In some embodiments, openings 113 on collimator 112 are configured topass through radiation within a relatively narrow range of incidentangles (e.g., a is between 0 to about 10 degrees), while openings 123 oncollimator 122 are configured to pass through radiation within a widerrange of incident angles (e.g., β is between 0 to about 30 degrees, orbetween 0 to about 75 degrees). In some embodiments, the acceptableincident angle for collimator 122 is at least twice the acceptableincident angle for collimator 112. In an embodiment, the acceptableincident angle for collimator 122 is at least three times of theacceptable incident angle for collimator 112. In other words, FOV1 maybe designed to be narrower than FOV2. The allowable pass-through anglesof radiation may be controlled by tailoring various parameters such aslength, size, shape, and tilt orientation of each opening 113 or 123. Inan embodiment, collimator 112 has openings with higher aspect ratios(hole length over diameter or width) than collimator 122. For example,at least some openings 113 may have a higher aspect ratio of height towidth than some openings 123. The aspect ratio may be determined usingany suitable methods. As shown in FIG. 2, the aspect ratio of an opening123 may represent a ratio between a height (denoted as H) of opening 123and a width (denoted as W) of opening 123. In some embodiments, openings113 and/or 123 have irregular shapes. For example, an opening 113 mayhave a different width from another opening 113, and an opening 123 mayhave a different width from another opening 123. An opening 113 or 123itself may have a varying width. In such cases, the aspect ratio may bedetermined using flexible methods that are consistent with principlesdisclosed herein (i.e., the goal is for collimator 112 to pass photonsthat have a narrower range of incident angles than collimator 122). Sucha goal may be achieved, for example, if openings 113 are smaller and/ordeeper than openings 123. In some embodiments, patient 102 is positionedwithin a target volume 129 of imaging system 100, as illustrated in FIG.2. Target volume 129 is shown as a rectangle box in the cross-sectionalview of FIG. 2, but in a 3D view may take the form of a cylinder. Forexample, the target volume may be a cylinder with a diameter of 50 to 70cm. The target volume for specific body parts such as head might besmaller. In such cases, the design consideration for openings 113 and123 applies to some openings 113 whose FOVs overlap with some FOVs ofopenings 123 within target volume 129. Because the FOVs of openings 113and 123 grow wider when moving upward farther away from openings 113 and123, the concept of FOV overlapping specifically applies within targetvolume 129.

Openings 113 and 123—which may also be called tunnels, apertures, orpass-through features—may have any suitable shape, size, number, and/ordistribution within their respective collimators. In some embodiments,openings 113 may include parallel holes, fan beams, cone beams,slit-slat, pinholes, multi-pinholes, any other suitably shaped openings,or combinations thereof. In some embodiments, collimator 112 is placedvery close (e.g., 2 cm or less) to patient 102. Thus, collimator 112 mayuse parallel holes or fan-beams (converging or diverging) since suchfeatures do not need significant separation from patient 102. In someembodiments, openings 113 may slant, converge, or diverge and may formfan beams or cone beams, etc. Openings 123 may include parallel holes,fan beams, pinholes, multi-pinholes, any other suitably shaped openings,or combinations thereof. In an example, openings 123 include a pluralityof pinholes, where the number of pinholes may be greater than 11,greater than 23, or greater than 59, or greater than 100. For example, acommonly used coded aperture pattern, MURA (modified uniformly redundantarray) of sizes 5, 7, 11, and 13 comprise 12, 24, 60, and 84 holes,respectively. A higher number of pinholes helps improve imagingsensitivity. Further, openings 123 may be single pinhole, multi-pinhole,multiple pinhole modules (including spread field imaging (SFI) or codedaperture). In some embodiments, openings 123 may slant, converge, ordiverge and may form fan beams or cone beams, etc. In an embodiment,openings 113 and 123 have different shapes (e.g., openings 113 beingparallel holes, while openings 123 being multi-pinholes).

In some embodiments, collimators 112 and 122 are each made of narrowopenings separated by walls made of heavy metal called septum. As shownin FIG. 2, collimators 112 and 122 each include a perforated plate madeof heavy metal such as lead and tungsten. The thickness of the plate,depending on the energy of photons imaging system 100 is designed toimage, is large enough to stop the majority of the radiation so thatphotons primarily pass through the openings on the plate. For example,for the commonly used isotope, Technetium-99m (^(99m)Tc), emitting gammarays with energy around 140 keV, a thickness of 2 mm to 3 mm is usuallyenough for a plate made of lead, and a thickness of 1.5 mm to 2 mm fortungsten. The thickness may be greater to image higher energy gammarays. Photons pass through the openings in the collimator plate.Sometimes, the height of an opening, i.e., the thickness of thecollimator, is much larger than the opening width. For example, anopening width may be 2-3 mm, while an opening height may be greater than2 cm. Collimators 112 and 122 may be placed at certain distances fromtheir respective detectors to allow photons coming from the designed FOVpassing the openings to spread across the detector surface. In someembodiments, collimator 112 is placed very close to detector 114, e.g.,with no gap or with a gap equal to or less than about 2 cm. Incomparison, collimator 122 is placed relatively far away from detector124, e.g., with a gap of about 3 cm or greater, often more than 5 cm.The distance between the bottom surface of collimator 122 and a topsurface of detector 124 is large enough that photons pass throughneighboring openings may be received by the same overlapping area ondetector 124 (or some areas of detector 124 may receive photons passingthrough different openings). This is an effect often called photonmultiplexing. In an example, a distance between the bottom surface ofcollimator 122 and a top surface of detector 124 is at least half of adistance between patient 102 and the top surface of collimator 122.

In some embodiments, collimator 112 is coupled to detector 114 such thatthey move (e.g., rotate) together. In other embodiments, collimator 112and detector 114 may move individually with respect to each other.Similarly, collimator 122 and detector 124 may move together orindividually with respect to each other. Further, imager 110 and imager120 may be configured to move together or individually with respect toeach other. In an example, imager 110, or imager 120, or both may moveduring imaging while patient 102 remains still. Imager 120 may movewhile imager 110 remains still, or vice versa. The motion of an imager(e.g., imager 110 or 120) may include shifting laterally (in x or ydirections), or shifting vertically (in z direction), or tilting, orrotating, or combinations thereof. In some embodiments, when an imagermoves, images are captured in a “step and shoot” fashion including, forexample, the following operations: moving the imager to a firstposition, acquiring a first image from the first position for a periodof time, moving the imager to a second position, and acquiring a secondimage from the second position for a certain period of time. The imagersmay move about patient 102 to other positions to repeat the operations.

Collimator 112 may have various suitable shapes and dimensions. As shownin the top view of FIG. 1A, in this embodiment, collimator 112 has fourarms or portions including superior portion 112S, inferior portion 112I,left portion 112L, and right portion 112R, which together surround anempty center portion. Superior portion 112S and inferior portion 112I ofcollimator 112 correspond to the superior and inferior sides of patient102, respectively. Superior portion 112S and inferior portion 112I areseparated by the empty center portion, which allows radiation to reachcollimator 122. Although FIG. 1A illustrates collimator 112 as havingfour portions located along the peripheral of an FOV of collimator 122),collimator 112 may take any suitable configurations in variousembodiments. In an embodiment, collimator 112 may be of one continuouspiece that fully surrounds the peripheral of the FOV of collimator 122.In another embodiment, collimator 112 may include discrete (or disjoint)portions that are located along the peripheral of the FOV of collimator122. To further this embodiment, collimator 112 may have any suitablenumber of portions or sides such as one side, two sides, three sides,four sides, or more than four sides. For example, collimator 112 mayinclude one or more of the superior portion 112S, inferior portion 112I,left portion 112L, and right portion 112R. Note that the orientations(superior, inferior, left, and right) are used herein to represent thefour sides of collimator 112. The relative position with regard to asubject may change as the camera/collimator module rotates around thesubject. In medical imaging, as patients usually have bigger length(height) than width, in some embodiments, collimator 112 may includeonly two portions, which usually are portions 112S and 112I. Eachportion of collimator 112 may have a regular shape or an irregular shapeand may have curved or straight edges. Further, the discrete portions ofcollimator 112 may be placed at any appropriate locations along theperipheral of the FOV of collimator 122. For example, the portions 112Sand 112I may be placed proximate to the head and foot of the subject,while the portions 112L and 112R may be placed proximate the left andright sides of the subject, or proximate the front and back sides of thesubject. Still further, the portions 112L and 112R may be placedsymmetrically or asymmetrically with respect to a centerline from thehead to the foot of the subject.

Collimator 112 is coupled to one or more detectors 114, which may recordphoton counts for imaging. Information collected by detector 114increases imaging sensitivity because photons from more areas aredetected than using detector 124 alone. The overall size of imager 110fits in a space between patient 102 and imager 120. Within imager 110,detector 114 may have any suitable size that allows it to fit in a spacebetween collimators 112 and 122. In an embodiment, detector 114 has arelatively compact size or small footprint such that its thickness(denoted as T1 in FIG. 1B) is less than a distance between a bottomsurface collimator 112 and a top surface of collimator 122, and that itswidth is not greater than a width of collimator 112 (that is, in a topview such as FIG. 1A, detector 114 is hidden underneath collimator 112).The compact size helps reduce costs and/or system size. In someembodiments, detector 114 is a direct conversion detector that convertsX- or gamma ray photons directly into electrical signals. One type ofsuch direct conversion detectors is semiconductor detector (i.e., asemiconductor-based detector), such as one based on cadmium telluride(CdTe), cadmium zinc telluride (CZT), or high purity germanium (HPGe).Collimator 112 may also be a scintillator coupled with compact photomultiplier tubes (PMTs), silicon photomultiplier tubes (SiPMT), oravalanche photodiodes. Each portion or side of collimator 112 may beassociated with (or coupled to) a dedicated detector 114. Alternatively,multiple portions of collimators 112 may share a detector 114. On theother hand, within imager 120, detector 124 may be of a different typethan detector 114. In an embodiment, detector 124 is a scintillator(such as sodium iodide (NaI) or caesium iodide (CsI) based) detector.

In addition to filtering detecting photons, imager 110 (includingcollimator 112 and detector 114) may serve other purposes. For example,imager 110 acts as an FOV limiter, which limits the area that can beseen by collimator 122. As shown in FIG. 2, the FOV of collimator 112(FOV1 within lines A1 and A2) overlaps with the FOV of collimator 122(FOV2 within lines B1 and B2) in an overlapping volume or region. Inother words, if lines A1, A2, B1, and B2 are extended upwards, lines A1and B1 would intercept each other over the left side of collimator 112,and lines A2 and B2 would intercept each other over the right side ofcollimator 112). In an embodiment, imager 110 is disposed on theperipheral of FOV2 such that imager 110 partially blocks FOV2. Forexample, imager 110 may partially block the FOV of one or more openings123 that are disposed closest to imager 110. As illustrated in FIG. 2,superior portion 112S of imager 110 blocks the volume of space betweenlines B1 and C1 from the left-most opening 123 on collimator 122, whereline C1 connects the top-right corner of superior portion 112S and thebottom-right corner of the left-most opening 123 on collimator 122.Similarly, inferior portion 112I of imager 110 blocks the volume ofspace between lines B2 and C2 from the right-most opening 123 oncollimator 122, where line C2 connects the top-left corner of inferiorportion 112I and the bottom-left corner of the right-most opening 123 oncollimator 122. On the other hand, to allow photons to reach collimator122, the empty portion of collimator 112 is outside FOV1 but withinFOV2. In that sense, imager 110 serves as a view finder for imager 120.Because openings 123 have wider acceptable incident angles than openings113, line C1 would intercept line A1 if extended far enough. In someembodiments, even the FOV of the leftmost opening 113 in collimatorportion 112S (or the rightmost opening 113 in collimator portion 112I,as shown in FIG. 2) partially overlaps with the FOV of collimator 122within target volume 129. Further, collimator 122 may not be in FOV1.Thus, photons that have passed through openings 113 in collimator 112 donot pass through openings 123 in collimator 122. For example, as shownin FIG. 2, a photon travelling along line D1 or line D2 (after passingthrough an opening 113) may not pass any opening 123. Such aconfiguration allows collimators 122 and/or 112 to have a smallerfootprint (e.g., width and/or length) than conventional collimators,thereby reducing system cost. One design is to mix high-cost andlow-cost detectors for collimators 112 and 122 to subsequently reducethe overall cost of imaging system 100. In one example, detector 114 isa high-cost, high-performance detector such as semiconductor-baseddetectors (e.g., CZT detector), and detector 124 is a scintillator-baseddetector, or vice versa. In this case, the overall cost of detectors 114and 124 is lower than the cost of using CZT detectors for both detectors114 and 24.

The imaging modality used by imaging system 100 may include gammacamera, SPECT (single photon emission computed tomography), and PET(positron emission tomography), and other suitable ones. In SPECTimaging, for instance, one or more collimators 112 is placed betweendetector 114 and patient 102, and openings 113 on collimator 112 maydetermine the directions and angular span from which radiation can passthrough collimator 112 to reach a certain position on detector 114.Similarly, one or more collimators 122 may be placed between detector124 and patient 102, and openings 123 on collimator 122 may determinethe directions and angular span from which radiation can pass throughcollimator 122 to reach a certain position on detector 124. In someembodiments, collimator 122 and detector 124 are connected or fixedtogether via coupling component 126, which also provides shielding toprevent photons passing through the space between 122 and 124. In SPECTimaging, cameras may be rotated to acquire 2D images from differentangles. In an embodiment, collimator 122 and detector 124 may move(e.g., rotate) together. Parts 110 and 120 may also be connected so thatthey rotate together to acquire images.

Data acquired from imager 110 can be used to reconstruct an image ofFOV1 independently. Further, data from subparts of imager 110 (e.g.,collimator portions 112S, 112I, 112L, or 112R) may be reconstructedindependently as well. Various methods including filtered backprojection, algebraic, or statistical methods (such as an expectationand maximization (EM) method or an ordered subset expectationmaximization (OSEM) method) may be used for image reconstruction with aparallel collimator. In some embodiments, image signals (sometimescalled counts) from both imager 110 and imager 120 can be used togetherin image reconstruction. For instance, data acquired from imager 120 canbe used to reconstruct an image of FOV2 using an iterative method. Theiterative method utilizes image data reconstructed in FOV1 from imager110 as an initial guess for the overlapping volume of FOV1 and FOV2. Insome embodiments, signal(s) from imager 110 can be reconstructed togenerate a cross-sectional image of FOV1 (denoted as F1), which may haverelatively low resolution. Because FOV1 and FOV2 overlap, part of thereconstructed image F1 that represents a region overlapping with FOV2may be used as initial estimate for reconstruction of the overlappingregion. The overlapping region may be updated sequentially with imagereconstruction using data from imager 110 and from imager 120.Alternatively, the overlapping region may be updated simultaneously withimage reconstruction using data from images 110 and 120 together.

Image reconstruction may use system 100 and other systems or componentscoupled therewith. FIG. 3 illustrates an example molecular or nuclearimaging system 300, which includes imaging system 100, a gantry 310, aplatform 312, a control console 320, and a computer system 330. In thepresent embodiment, computer system 330 includes a data storage unit332, an image processor 334, an image storage unit 336, and a display338. One or more imaging modules (e.g., imaging system 100 includingimagers 110 and/or 120) are mounted on gantry 310, which may move,rotate, and acquire data simultaneously. Patient 102 is placed on aplatform 312 (e.g., a couch) for examination or treatment by imagingsystem 100. In some embodiments, imagers 110 and/or 120 are coupled togantry 310 through movable parts so that they may move (e.g., rotate) ongantry 310.

Imagers 110 and/or 120 may detect and record radiation emitted frompatient 102 and transfer recorded information to data storage unit 332.Then, image processor 334 may use the recorded information toreconstruct volumetric images representing radiopharmaceuticaldistributions within patient 102. The reconstructed images are stored inthe image storage unit 336, which can be manipulated and displayed ondisplay 338 for viewing. Control console 320 may be used by an operatoror technician to control imaging system 100 in acquiring data. In someembodiments, control console 320, data storage unit 332, image processor334, image storage unit 336, and display 338 are integrated in acomputer system 330. In some embodiments, one or more computercomponents (such as control console 320, data storage unit 332, imageprocessor 334, image storage unit 336, and display 338) can be partiallyor entirely located at a remote location (e.g., on the cloud). In someembodiments, one of more of these components may exist locally orremotely.

The present disclosure provides distinctive methods for nuclear medicineand molecular imaging to accommodate a collimator such as collimator122, which may have a plurality of pinholes (called a multi-pinholecollimator). Image reconstruction methods may use equations (1), (2),(2-1), and (2-2), described below. An example method in imagereconstruction is called a maximum likelihood expectation andmaximization (MLEM) method, which estimates object images usingequation:

$\begin{matrix}{{\hat{f}}_{j}^{({k + 1})} = {\frac{{\hat{f}}_{j}^{(k)}}{\sum\limits_{i = 1}^{I}K_{ij}}{\sum\limits_{i = 1}^{I}\frac{p_{i}K_{ij}}{\sum\limits_{j = 1}^{J}{K_{ij}{\hat{f}}_{j}^{(k)}}}}}} & (1)\end{matrix}$

where {circumflex over (f)}_(j) ^((k+1)) is the (k+1)th estimate of thejth element of f, the object image, p_(i) is the ith element of ameasured image, and K_(ij) is a transition matrix representing theprobability of photon emitted from jth element of the object beingdetected by the ith element of a detector. Let p_(r) be the denominatorinside the second summation: p_(r)=Σ_(j=1) ^(J)K_(ij){circumflex over(f)}_(j) ^((k)), often called a forward projection representing anexpectation of measured image based on the kth estimated object image{circumflex over (f)}^((k)).

In some cases such as when a coded aperture mask is used as acollimator, equation (1) can be further written as

$\begin{matrix}{{\hat{f}}^{({k + 1})} = {{\hat{f}}^{(k)} \times ( {h \otimes \frac{p_{c}}{{\hat{f}}^{(k)}*h}} )}} & (2)\end{matrix}$

where h is a coded aperture mask shadow, and p_(c) is a measured imageafter correction for angular effects. p_(c)=p*Cc, where Cc is theangular effect correction factor, and * and ⊗ represent convolution andcorrelation operations. The coded aperture mask is a plate (made ofheavy metal such as lead and tungsten) with multiple pinholes.Representing the mask, h is a matrix. In the original form, h is amatrix of “0”s and “1”s, where each element corresponds to a gridposition on the plate, 1 represents an opening (pinhole) at thatposition, and 0 represents otherwise. This matrix can be magnified torepresent the magnifying effect of the mask shadow projected by a sourceon the detector, and interpolation may be used to calculate the matrixelement values.

Equation (2) is suitable for a thin imaging subject whose thickness ismuch smaller than the distance between the subject and a collimator suchas collimator 122. For thicker subjects, a three-dimensional (3D) methodis used. For example, a subject image at distance z can be estimatedusing the following equation:

$\begin{matrix}{{{\hat{f}}^{({K + 1})}(z)} = {\frac{{\hat{f}}^{(K)}(z)}{\sum\limits_{x,y}{h(z)}}\lbrack {{h(z)} \otimes \frac{p_{c} - {\sum\limits_{z \neq z^{\prime}}{{{\hat{f}}^{(K)}( {z\; \prime} )}*{h( {z\; \prime} )}}}}{{{\hat{f}}^{(K)}(z)}*{h(z)}}} \rbrack}} & ( {2\text{-}1} )\end{matrix}$

A slightly different formula can be used to estimate the subject aswell:

$\begin{matrix}{{{\hat{f}}^{({K + 1})}(z)} = {\frac{{\hat{f}}^{(K)}(z)}{\sum\limits_{x,y}{h(z)}}\lbrack {{h(z)} \otimes \frac{p_{c}}{\sum\limits_{z}{{{\hat{f}}^{(K)}(z)}*{h(z)}}}} \rbrack}} & ( {2\text{-}2} )\end{matrix}$

The present disclosure provides imaging techniques that, depending onthe application, may be used for general purpose imaging or for imagingcertain body parts of a subject such as patient 102. For example,systems disclosed herein may be used for the imaging of human breast(s).FIGS. 4A and 4B illustrate an example molecular breast imaging (MBI)system 400, which may be designed to acquire images of a breast 104(located on the chest 106 of patient 102) for screening or diagnosis ofdiseases such as breast cancer. Specifically, FIG. 4A is a schematic topview of one MBI system 400, and FIG. 4B is a cross-sectional view of twoMBI system 400s, each deployed on a different side of breast 104 (e.g.,one underneath breast 104, and another above breast 104). In someembodiments, each MBI system 400 in FIG. 4B may be considered an imagingunit of the overall (bigger) MBI system shown in FIG. 4B. In practice,depending on factors such as costs, one or more such imaging units maybe used on breast 104. In the interest of clarity, the followingdescriptions focus on one MBI system 400, with the understanding thatsimilar principles may be used for the other MBI system 400 (note,however, that differences may exist between two MBI systems used ondifferent sides of breast 104).

In the embodiment shown in FIGS. 4A and 4B, each imaging system 400 is ahybrid system including two imaging modules or parts (called imagers)410 and 420. Imager 410 includes first collimator 412 and first detector414, and imager 420 includes second collimator 422, second detector 424.Imaging system 400 may include other parts such as connectors thatcouple parts together, motors that cause parts to move, photon shieldingcomponents, a housing component that contains other parts, etc. Forexample, a coupling and shielding component 426 may connect collimator422 and detector 424 such that collimator 422 and detector 424 move(e.g., rotate) together, and also provide shielding around imager 420.Since one skilled in the art would recognize that various aspects of MBIsystem 400 are similar to imaging system 100 and that variouscharacteristics of imaging system 100 would similarly apply tocounterparts in MBI system 400, such similar aspects are not repeatedbelow in the interest of conciseness. On the other hand, variouscharacteristics of MBI system 400 can also be used in imaging system 100where applicable.

Compared with conventional screening method such as mammography, MBIsystem 400 demonstrates higher sensitivity in lesion detection in thecase of dense breast. In use, radiopharmaceuticals (such as technetiumTc 99m (99m Tc) sestamibi) may be injected into patient 102 a fewminutes before image acquisition. The radiopharmaceuticals may getdistributed in breast 104 as well as other parts of patient 102 (e.g.,in a chest area close to breast 104, which may cause noises). Duringacquisition, breast 104 may be compressed to reduce its thickness forbetter penetration of gamma photons as well as to reduce its motion.

In an embodiment of MBI system 400, collimators 412 and 422 are placedon a same side of breast 104, but collimator 422 is positioned furtheraway from breast 104 than collimator 412 (in the vertical direction Z ofFIG. 4B). In addition, collimator 412 and detector 414 are placed closerto chest 106 than collimator 422 (in the horizontal direction X of FIG.4B). In an embodiment, collimators 412 and 422 each include openings forpassing through radiation emitted from breast 104. To reduce noises fromother body parts not intended to be imaged (such as noises from areas ofchest 106 that are close to breast 104), collimator 412 may havenarrower and/or deeper openings than collimator 422. For instance,openings in collimator 412 may have a higher aspect ratio (of height towidth, as described above with respect to imaging system 100) thanopenings in collimator 422 such that only photons with a narrow incidentangle range may pass through collimator 412 to reach detector 414. Sincecollimator 412 is positioned close to chest 106, its openings mayeffectively block out noise emissions from areas of chest 106 that areclose to breast 104. In some embodiments, the openings in collimator 422and the positioning of collimators 412 and 422 on one side (e.g., lowerside) of breast 104 are designed to ensure that radiation originatedfrom above or to the left of collimator 412 positioned on the oppositeside (e.g., upper side) of breast 104 will be blocked by shielding 416and collimator 412 (on the upper side), as illustrated by the line E1 inFIG. 4B. In the case of only one imaging system 400 being employed,there may be a radiation shielding device positioned in such a way toensure that. In an embodiment, openings in collimator 412 includeparallel holes, and openings in collimator 422 include pinholes.

Although MBI system 400 as shown in FIGS. 4A and 4B includes multiplecollimators for imaging breast 104, it is understood that, in someembodiments, MBI system 400 may use a single collimator such ascollimator 422, which works with other parts or has distinctive featuresto implement improved imaging of breast 104. Such distinctive featuresmay also be used in the multiple collimator context to further improveimaging of breast 104. FIGS. 4C and 4D illustrate an example MBI system450 with one or more FOV restrictors 418. For example, FOV restrictors418 made of heavy metal may be placed between breast 104 and collimator422. In an embodiment, FOV restrictor 418 is located on the side ofcollimator 422 that is proximal to chest 106 (that is, FOV restrictor418 is closer to chest 106 than collimator 422). FOV restrictor 418 maybe positioned similar to imager 410 in FIGS. 4A and 4B, and may replaceimager 410 when image data from imager 410 is not needed in someapplications. FOV restrictor 418 (sometimes called a view finder, asdescribed above) may be positioned partially inside the FOV ofcollimator 422 so that the FOV restrictor blocks some photons fromreaching collimator 422 that would otherwise reach collimator 422. Theposition of an FOV restrictor 418 may change during imaging, forexample, in combination with imager motion, which is described furtherbelow.

In some embodiments, one or more radiation shielding devices, including416 and 418 which replaces the FOV restrictor 418, may be placed on bothsides of breast 104 against chest 106, as shown in FIGS. 4C and 4D.Radiation shielding devices 416 and 418 may help block radiation fromhitting collimator 422, which may have multiple pinholes capable ofreceiving photons with a wide incident angle range. The optionalradiation shielding device 418 may replace collimator 412 and detector414 (in case image data from detector 414 is not needed) or may bedeployed in addition to collimator 412 and detector 414. Even in asingle-collimator MBI system 400, radiation shielding devices 416 and/or418 may be placed on both sides of breast 104. For example, radiationshielding device 416 and/or 418 placed on the upper side of breast 104may still help block photons traveling from above breast 104 or fromchest 106 and prevent the photons from hitting collimator 422 placed atthe lower side of breast 104. Although radiation shielding devices 416and/or 418 are described with respect to MBI system 400, it should beunderstood that one or more similar radiation shielding devices may alsobe used in imaging system 100 (e.g., in addition to or in place ofimager 110) in order to restrict the FOV of collimator 122.

In some embodiments, MBI system 400 is a breast gamma imaging systemthat employs one or more collimators having multi-pinholes. Comparedwith parallel holes, pinholes provide various benefits for breastimaging such as improving imaging resolution and sensitivity.Conventional MBI systems do not use multi-pinhole collimators because ofthe complexity of the image reconstruction algorithm and wide incidentangles that allow photons from chest area to pass through and thereforedegrade image quality. For example, pinholes can receive photons with awider incident angle range. In an embodiment, collimator 422 usingmulti-pinholes is capable of receiving photons with an incident angle(β) equal to or greater than 30, 45, 60 degrees. In addition, themultiple pinholes may not be positioned on an equal spacing grid (thatis, the pinholes may or may not be equally spaced on the plate ofcollimator 422). In addition, the height of pinholes (usually equal tothe thickness of collimator plate) may be smaller than the height ofparallel holes in a parallel-hole-based collimator. In an embodiment,collimator 422 using multi-pinholes is about 1 mm to about 3 mm thick.Furthermore, the number of pinholes may be quite large (e.g., more than100). In an embodiment, the multi-pinhole collimator may be a codedaperture collimator. The multi-pinhole collimator may include aperturesof different shapes or sizes.

In some embodiments, MBI system 400 includes one or more imagers thatmay move during imaging while patient 102 and breast 104 remain still.The motion of an imager (e.g., imager 410 or 420) may include shiftinglaterally (in x or y directions), or shifting vertically (in zdirection), or tilting, or rotating, or combinations thereof. In someembodiments, when an imager moves, images are captured in a “step andshoot” fashion including, for example, the following operations: movingthe imager to a first position, acquiring a first image from the firstposition for a period of time, moving the imager to a second position,and acquiring a second image from the second position for a certainperiod of time. The imager may move about breast 104 to other positionsto repeat the operations. In some embodiments, during the motion, adistance between collimator 422 and detector 424 remains constant(unlike certain systems where the distance changes). In someembodiments, the distance between collimator 422 and detector 424 islarge enough so that there is always multiplexing, i.e., photons maypass through different openings to reach the same surface area ofdetector 424; that is, images projected through the apertures aresubstantially overlapped. In an embodiment, at least half of imagesprojected through an aperture overlap with images through at least oneother aperture. In some embodiments, a 3D image of the breast comprisingof multiple image slices (which represent object slices at differentdepths) is reconstructed based on one or more acquired images, and the3D image is presented for viewing. This is beneficial because thecontrast of lesion is higher in its respective slice of a 3D image thanin a 2D image of the breast. In an embodiment, the number of slices isequal to or greater than the number of images acquired.

Although MBI system 400 is described with respect to molecular breastimaging, it is understood that principles disclosed herein apply toother breast imaging techniques using radiopharmaceuticals such asbreast specific gamma imaging (BSGI) or scintimammography (SMM).

Referring now to FIG. 5, a flow chart of a method 500 for examining asubject using molecular imaging is illustrated according to variousaspects of the present disclosure. The method 500 is merely an exampleand is not intended to limit the present disclosure to what isexplicitly illustrated in the method 500. Additional operations can beprovided before, during, and after the method 500, and some operationsdescribed can be replaced, eliminated, or moved around for additionalembodiments of the method. The method 500 is described below inconjunction with FIGS. 1-4.

At operation 510, a first collimator (e.g., collimator 112 or collimator412) filters photons emitted from a subject such as patient 102, and asecond collimator (e.g., collimator 122 or collimator 422) filters otherphotons emitted from the subject. In an embodiment, the first and secondcollimators are positioned such that their fields of view partiallyoverlap in an overlapping volume. At operation 520, a first detector(e.g., detector 114 or detector 414) detects photons that pass throughthe first collimator, and a second detector (e.g., detector 124 ordetector 424) detects other photons that pass through the secondcollimator.

At operation 530, an image processor (e.g., image processor 334)reconstructs a medical image of the subject based on photons detected byboth the first and second detectors. In an embodiment, a first partialimage is reconstructed based on photons that pass through theoverlapping area and are detected by the first detector, and a secondpartial image is reconstructed based on photons detected by the seconddetector. The reconstruction of the second partial image optionally usesthe first partial image as an initial estimate for the overlappingvolume. Further, the reconstruction of the second partial image mayupdate the first partial image using an iterative method. In anotherembodiment, an image of combined FOVs of first and second collimatorsmay be reconstructed using photons detected by the first and seconddetectors simultaneously, and methods such as an MLEM method given inequation (1) herein may be used for reconstruction.

Although not intended to be limiting, one or more embodiments of thepresent disclosure provide many benefits for molecular imaging of asubject such as a patient. For example, the hybrid structure of multipleimagers allows a first imager (e.g., imager 110 or 410) to pick upperipheral photons (which increases imaging sensitivity), while reducingnoises to be received by a second imager (e.g., imager 120 or 420).Therefore, system performance is improved.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure. Accordingly, it isappropriate that the appended claims be construed broadly and in amanner consistent with the present disclosure.

What is claimed is:
 1. A medical imaging system, comprising: a firstcollimator configured to filter radiation emitted from a subject; afirst detector configured to detect radiation that has passed throughthe first collimator; a second collimator configured to filter radiationemitted from the subject, wherein the first collimator partially blocksa field of view (FOV) of the second collimator; and a second detectorconfigured to detect radiation that has passed through the secondcollimator.
 2. The medical imaging system of claim 1, wherein the seconddetector and the second collimator are configured to move together. 3.The medical imaging system of claim 1, wherein the first collimatorcomprises first openings for passing through radiation with a firstaspect ratio of height to width, wherein the second collimator comprisessecond openings for passing through radiation with a second aspect ratioof height to width, and wherein the first aspect ratio is higher thanthe second aspect ratio.
 4. The medical imaging system of claim 3,wherein the first openings of the first collimator include parallelholes.
 5. The medical imaging system of claim 3, wherein the secondopenings of the second collimator include multiple pinholes in a plate.6. The medical imaging system of claim 5, wherein a number of thepinholes on the second collimator is 11 or greater.
 7. The medicalimaging system of claim 1, wherein the second collimator is positionedat least 3 cm away from the first collimator.
 8. The medical imagingsystem of claim 1, wherein the first collimator is positioned on aperipheral of the FOV of the second collimator.
 9. The medical imagingsystem of claim 1, wherein the first collimator includes discreteportions along the peripheral of the FOV of the second collimator. 10.The medical imaging system of claim 9, wherein the discrete portions oneor more of: a superior portion, an inferior portion, a left portion, anda right portion.
 11. The medical imaging system of claim 1, wherein thefirst detector is fully positioned between the first collimator and thesecond detector.
 12. The medical imaging system of claim 11, wherein thefirst detector is a direct conversion detector.
 13. The medical imagingsystem of claim 11, further comprising: an image processor configured toreconstruct a medical image of the subject based on radiation detectedby both the first detector and the second detector; and a display deviceconfigured to display the reconstructed medical image.
 14. A breastimaging system for examining a breast on a chest of a patient,comprising: a first collimator configured for placement proximal to thechest; and a second collimator configured for placement distal to thechest, wherein the first and second collimators are placed on a sameside of the breast, and wherein the second collimator is positionedfurther away from the breast than the first collimator.
 15. The breastimaging system of claim 14, wherein the first and second collimatorscomprise first and second openings, respectively, for passing throughradiation emitted from the patient, wherein the first and secondopenings have first and second aspect ratios, respectively, of height towidth, and wherein the first aspect ratio is higher than the secondaspect ratio.
 16. The breast imaging system of claim 15, furthercomprising: a third collimator for placement proximal to the chest,wherein the third collimator also comprises openings for passing throughradiation with the first aspect ratio of height to width; and a fourthcollimator for placement distal to the chest, wherein the fourthcollimator also comprises openings for passing through radiation withthe second aspect ratio of height to width, wherein the third and fourthcollimators are positioned on a same side of the breast that is oppositeto the side of the breast where the first and second collimators areplaced on.
 17. The breast imaging system of claim 15, wherein theopenings of the first collimator include parallel holes.
 18. The breastimaging system of claim 15, wherein the openings of the secondcollimator include multiple pinholes.
 19. The breast imaging system ofclaim 14, further comprising: a first detector configured to detectradiation that has passed through the first collimator; and a seconddetector configured to detect radiation that has passed through thesecond collimator.
 20. The breast imaging system of claim 19, whereinthe second collimator and the second detector are configured to moveabout the breast while maintaining a constant distance between thesecond collimator and the second detector.
 21. A method for examining asubject using molecular imaging, the method comprising: filtering, by afirst collimator, photons emitted from the subject; detecting, by afirst detector, photons that pass through the first collimator;filtering, by a second collimator, photons emitted from the subject;detecting, by a second detector, photons that pass through the secondcollimator; and reconstructing, by an image processor, a medical imageof the subject based on photons detected by both the first and seconddetectors, wherein the first and second collimators are positioned on asame side of the subject.
 22. The method of claim 21, wherein the firstand second collimators are positioned such that their fields of viewpartially overlap in an overlapping region.
 23. The method of claim 22,wherein the reconstructing of the medical image comprises:reconstructing a first partial image based on photons that pass throughthe overlapping region and are detected by the first detector; andreconstructing a second partial image based on photons detected by thesecond detector, wherein the reconstruction of the second partial imageuses the first partial image as an initial estimate for the overlappingvolume.
 24. The method of claim 22, wherein the reconstruction of thesecond partial image further updates the first partial image using aniterative method.